This invention relates to optical monitoring of processes and, in particular, to monitoring the emissions from a plasma glow discharge for determining the optimum endpoint of a process.
It has been recognized for a long time that the optical emission from a plasma glow discharge, as with any luminous source, is characteristic of that discharge. This fact has been used to determine the endpoint of a process step by detecting a change in the emission; for example, its intensity or color. U.S. Pat. No. 4,312,732, for example, discloses a system for detecting endpoint on the basis of the intensity of the light emitted by the glow discharge. When the intensity, as represented by a variable voltage, reaches some predetermined level, the process is terminated. U.S. Pat. No. 4,246,060 discloses a system for detecting a temporary uniformity of the voltage as indicative of optimal endpoint.
While these techniques are suitable for certain processes, there are many for which they are not. Further, the equipment maker has a fundamental problem of providing general purpose equipment. The alternative, making equipment suitable for only one process, e.g. etching a particular layer overlying a specific material using a certain gas mixture, is very expensive, both for the manufacturer and the consumer.
Thus, one needs to make equipment as flexible as possible. In addition, the sensitivity of the endpoint detection apparatus must be improved so that useful nuances can be detected in the glow discharge. For example, in an etch process where the underlying film has a relatively small exposed area, it is difficult to detect when the plasma has penetrated film. Thus the optical emissions may be too small to detect with prior designs or difficult to distinguish from noise.
For other processes, one may want to add, subtract, multiply, divide and/or differentiate the signals from more than one photocell. Providing any one of these functions is not particularly difficult. Providing all of them in as simple and least costly manner as possible is not as easy.
Another problem is system errors. In a plasma reactor having multiple photocells, one cannot be sure that the differences in intensity from various inputs are due to changes in the glow discharge. The location and transparency of the port may cause spurious readings, particularly if polymers or other materials accumulate on one port more quickly than on another. Also, pressure, power, or gas mixture changes can cause common mode changes in photocell outputs.
In view of the foregoing, it is therefore an object of the present invention to provide an improved apparatus for detecting changes in a luminous process.
Another object of the present invention is to provide apparatus for performing mathematical operations on analog signals from photocells to help detection of endpoint on films with low exposed areas.
A further object of the present invention is to provide apparatus which can be easily configured to perform a variety of analyses on analog signals from photocells.
Another object of the present invention is to provide endpoint detection apparatus which is less susceptible to system errors.
Another object of the present invention is to provide a normalization servomechanisms that ensures that each process generating an endpoint signal that is initialized at a level specified in the application recipe.